Felwinter's excels in one on one, but really falls off when dealing with two or more enemies in CQC thanks to its poor RoF. The only perk that natively buffs it's RoF is full auto, which cuts the ammo dump time in half. The issue with this perk is you are loosing out on shot package. I think it's fine as is. There are natural trade offs for the incredibly high impact. The only combo which could be considered OP is shot package + hammer forged. Even the Vanguard shotty has this roll and despite having a lower overall impact, can still kill a Felwinter user just as easily.